1. Field of the Invention
The present invention relates to an internal pump adopted for an internal pump type boiling water reactor (hereinafter called the ABWR); Advanced Boiling Water Reactor), and particularly to an internal pump which is constituted to trap clad that enters the internal pump from a reactor pressure vessel.
2. Description of the Prior Art
A conventional internal pump will be explained with reference to FIGS. 1 to 3.
FIG. 1 is a vertical section schematically showing the ABWR. A reactor pressure vessel 1 accommodates coolant 2 and a core 3. The core 3 comprises a plurality of fuel assemblies (not shown), control rods 4, etc. In the reactor pressure vessel 1, there are arranged a shroud 5, a core support plate 6 and an upper grid plate 7, to support the core 3. The control rods 4 are driven by a control rod driving mechanism 8.
Above the core 3, a steam separator 9 is arranged, and a steam dryer 10 is disposed above the steam separator 9. Below the steam separator 9, a water supply pipe 11 is connected to the reactor pressure vessel 1. The water supply pipe 11 is connected to a water supply sparger 12.
Between the shroud 5 and the reactor pressure vessel 1, a downcomer portion 13 is defined. At a lower part of the downcomer portion 13, internal pumps 14 are circumferentially disposed at regular intervals. The internal pumps 14 forcibly circulate the coolant 2 toward the core 3. A reference mark 15 represents main steam piping.
FIG. 2 shows structure of the internal pump 14. The internal pump 14 comprises a pump section 21 and a motor section 22. The pump section 21 comprises an impeller 23, a diffuser 24 disposed on the discharge side of the impeller 23, a diffuser ring 25, etc. The impeller 23 is connected to the motor section 22 through a pump shaft 26.
At a bottom of the reactor pressure vessel 1, nozzles la are arranged to protrude upward. The diffuser 24 is fixed to each nozzle la with a stretch tube 27.
The motor section 22 comprises a motor case 32 which accommodates a rotor 30 engaging integrally with the pump shaft 26, a stator 31, etc. A front end portion of the motor case 32 is inserted into the nozzle la and welded thereto. Coolant water via piping 33 circulates inside the motor case 32 to prevent the motor section 22 from burning. An auxiliary impeller 34 is fixed to a lower end of the rotor 30. A lower end of the stretch tube 27 engages with a stepped portion formed at an upper part of the motor case 32.
A purge water inlet 41 is formed at an upper part of the motor case 32. Through the purge water inlet 41, purge water is supplied into the motor case 32.
As also shown in FIG. 3, the purge water supplied into the motor case 32 passes through a space A defined between the pump shaft 26 and the stretch tube 27 and flows into a space B defined between the pump shaft 26 and the diffuser 24. By supplying the purge water in this way, contaminated particles in the reactor pressure vessel 1 are prevented from flowing into the motor case 32. At the same time, the purge water cools an upper part of the motor case 32. Due to such a cooling effect of the purge water, despite a high water temperature of about 300 C. in the reactor pressure vessel 1, a temperature inside the motor case 32 is suppressed to about 40 C. As a result, polymeric materials such as a coil insulation material of the stator 31 and a sealing rubber material of a secondary seal are prevented from deteriorating due to the high temperature.
However, the above-mentioned conventional structure of the internal pump has several drawbacks. Clad of large specific gravity generated in the reactor pressure vessel 1 may flow into the motor case 32 against a flow of the purge water. To prevent the clad of large specific gravity from entering the motor case 32, a flow rate of the purge water shall be increased. If the flow rate of the purge water is increased, however, the pump shaft 26 and stretch tube are subjected to a great temperature difference, i.e., large thermal stress to deteriorate soundness of the pump shaft 26 and stretch tube 27.
In this way, the conventional internal pump structure requires a flow rate of the purge water to be increased to prevent the clad of large specific gravity from entering into the motor case. If the flow rate of purge water is increased, however, the pump shaft and stretch tube are subjected to large thermal stress to deteriorate soundness of the pump shaft and stretch tube.